Materials

6061 vs 7075 aluminium for CNC machining

Both are common machining aluminiums, and the choice changes your part cost by 30–60% and your surface finish options entirely. Here is the decision rule.

6061 and 7075 dominate precision machined aluminium components, and the choice is usually made early, on the basis of a strength figure quoted out of context. That produces errors in both directions: over-specifying 7075 where 6061 would do, and specifying 6061 where the part will yield under load.

Side-by-side comparison

Typical published values for 6061-T6 and 7075-T6. Mill certificates vary a few percent, so specify the temper explicitly rather than assume it.

Property	6061-T6	7075-T6
Tensile strength (ultimate)	About 310 MPa (45 ksi)	About 570 MPa (83 ksi)
Yield strength (0.2% offset)	About 275 MPa (40 ksi)	About 505 MPa (73 ksi)
Hardness (Brinell, typical)	Around 95 HB	Around 150 HB
Machinability	Good; continuous, stringy swarf	Better; roughly 10 to 20 percent faster cutting
Anodising response	Clean, clear, takes dye evenly; bright silver	Coats but darkens and shifts yellow-brown
Corrosion resistance	Good, including marine exposure	Poor to moderate; weak at cut edges and in chlorides
Relative material cost	Baseline	Typically 1.5 to 2.5 times 6061
Weldability	Good	Poor; not welded in structural service

Stiffness is the same

Both alloys have the same elastic modulus, around 69 GPa. Young's modulus is a property of the aluminium lattice and is not meaningfully changed by alloying or temper. If a part fails a stiffness requirement, switching to 7075 buys nothing and roughly doubles material cost. Alloy selection helps only when the failure mode is yielding: permanent deformation or fracture.

When 6061-T6 is the right answer

• Enclosures, brackets, plates, fixtures and housings where loads are modest and corrosion resistance matters more than strength.
• The part will be anodised and colour must be consistent, or the finish is part of the appearance.
• The part will be welded, or belongs to a welded assembly.
• Service includes moisture, salt or cleaning chemicals and you do not want to pay for a coating.
• Lead time and cost are constrained. 6061 is stocked in far more sizes and thicknesses.
• The design is not frozen. 6061 is more forgiving of rework.

When 7075-T6 earns its price

• High load in a small envelope. Aerospace brackets, UAV fittings, robotics end-effectors, racing parts and mould tooling where the section cannot grow.
• Weight-critical parts. Higher strength per unit mass lets you remove material 6061 would need, and the saving can exceed the price difference in a moving assembly.
• Wear surfaces. Higher hardness resists galling on pins, jigs and slide plates.
• Thin-wall parts. Better chip formation makes deep pockets and fine ribs more reliable.
• Fatigue-loaded components that must survive cycling, not a single peak load.

The 7075 anodising problem, in practice

This generates the most complaints after delivery, and it is nearly always a specification gap rather than a defect. 7075 contains roughly 5.1 to 6.1 percent zinc and 1.2 to 2.0 percent copper. Those elements do not form the clean, clear oxide that 6061 does. The coating still forms and protects, but the surface is noticeably darker, greyer, often with a yellow or brownish tint against the bright silver of anodised 6061. Hard anodising amplifies it, and dyed colours come out muddier and less reproducible.

• Two batches will not match. Mixed 6061 and 7075 parts in one assembly, anodised the same colour, will not look alike.
• Approve a colour sample in the actual alloy and temper, and keep it as the reference for later lots.
• Expect variation between lots even for one alloy. State an acceptable range, or move appearance-critical parts to 6061.

Corrosion: 7075 needs protection

6061 is a magnesium-silicon alloy with good general corrosion resistance, including marine exposure, and performs acceptably bare indoors. 7075 is a zinc-copper-magnesium alloy, and that composition is both why it is strong and why it corrodes. It is more susceptible to atmospheric and chloride attack, especially at cut edges and in crevices where moisture cannot dry.

7075 is also sensitive to stress corrosion cracking in some tempers. T73 trades a little strength for much better resistance and suits parts under sustained tension in a corrosive environment. If a drawing says 7075-T6 for a wet or salt-exposed application, question it before the material is cut.

Any 7075 part used outdoors, in wash-down conditions or near the coast should carry a protective finish: anodising for general protection, chromate conversion coating where bonding or paint adhesion matters, or a qualified primer and paint system for severe exposure. Do not accept bare 7075 on the assumption that aluminium does not corrode.

6061-T6 versus 6061-T651

Buyers often read T651 on a mill certificate as an upgrade. It is a different condition, not a better one: the difference is residual stress, not strength. T6 means solution heat treated and artificially aged. T651 means the same plus a controlled stretch of roughly 1 to 3 percent after quenching to relieve internal stress. Minimum specified tensile and yield strengths are essentially identical.

What differs is dimensional stability after machining. When stressed plate is cut, residual stresses redistribute and the part moves: bowing in long thin parts, lost flatness in large plates, a part that measures correctly on the machine but not on the inspection table. T651 reduces this and suits heavily machined plates, long or asymmetric parts with flatness callouts, and parts referenced off a machined face in a second operation. For small, symmetrical parts, T6 is adequate and usually cheaper.

A decision rule for procurement

1. Start with 6061-T6. It covers most machined components at lower cost with better corrosion and finishing behaviour.
2. Move to 7075-T6 only if the part yields or fractures in 6061, must be smaller or lighter, or hardness is functional.
3. Do not move to 7075 for stiffness or deflection. The modulus is identical and the change buys nothing.
4. If the part is appearance-critical or dyed, stay with 6061.
5. If 7075 is confirmed, add surface treatment to the drawing at the same time. Bare 7075 outdoors is an incomplete specification.
6. If the part is a large or thin plate, specify 6061-T651 and treat stretching as a stability requirement.

The alloys are not competitors in most designs; they occupy different load regimes. Treat the material callout as a consequence of the load case, the finish requirement and the service environment, write all three into the drawing, and the choice becomes a short, defensible decision rather than a preference carried over from a previous project.